I would sew the two colors into 6" finished (so 6.5" unfinished) strip sets, then use a triangle ruler to cut into right triangles. As you're cutting with the triangle ruler, you'll rotate it so every other triangle has the opposite color in the corner:
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Another option would be to sew your strip set as above, then take your 12.5" white strip, sew along the top and along the bottom (right sides together), then cut your triangles, unfold and press and voila, you would have 2 sets of blocks and you would arrange accordingly:
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